Cathode ray tube having cylinder with internal resistive helix

ABSTRACT

A cathode ray tube housing an electron gun assembly mounted within a sealed evacuated envelope formed substantially of electrically insulating material has a first coating of electrically conductive material on part of its external surface and a second coating of electrically conductive material on part of its internal surface. The conductive coatings forming a capacitor. To minimize the effects of flashover, this capacitor is electrically connected to an electrode of the electron gun assembly by a resistor included in the electron gun assembly.

United States Patent [1 1 Grimmett et al.

[4 1 Sept. 30, 1975 CATHODE RAY TUBE HAVING CYLINDER WITH INTERNAL RESISTIVE HELIX [75] Inventors: Michael Charles Grimmett; Douglas Gerald Crawley, both of London, England [73] Assignec: Thorn Electrical Industries Limited,

London, England 221 Filed: July 3, 1973 [21] Appl. No.: 376,166

[30] Foreign Application Priority Data July 5. 1972 United Kingdom 31514/72 52] US. Cl 313/450; 313/451 51] Int. Cl. H01J 29/56; H01J 29/62 [58] Field of Search .1 313/83 SP [56] References Cited UNITED STATES PATENTS 3,295,008 12/1966 Gallaro et al. 315/3 Schwartz ct a1 313/82 R 3,375,390 3/1968 Schlesinger 313/83 SP FOREIGN PATENTS OR APPLICATIONS 1,211,898 11/1970 United Kingdom 313/83 SP Primary Examiner-Robert Sega] Attorney, Agent, or Firm--Robert F. OConnell 5 7 1 ABSTRACT 1 Claim, 2 Drawing Figures 'ffff 37 I5 43 1 X L\ x US. Patent Sept. 30,1975

NV Q a CATHODE RAY TUBE HAVING CYLINDER WITH INTERNAL RESISTIVE HELIX BACKGROUND OF THE INVENTION his invention relates to cathode ray tubes, and especially to cathode ray tubes including means for reducing the effects of internal flashover.

A cathode ray tube having an electron gun assembly mounted within a sealed evacuated envelope formed substantially of electrically insulating material is usually provided with a first coating of electrically conductive material on part of its external surface and a second coating of electrically conductive material on part of its internal surface. These coatings form a capacitor which can be used in a smoothing circuit for a high voltage supply circuit for use with the tube. This capacitor then contains a charge during operation of the tube, the internal coating being supplied with a positive voltage, and the external coating being connected to earth. Anodes are normally maintained at the same voltage as the internal coating in operation. A flashover from one of the anodes to one or more of the other electrodes, even the heater, having lower voltages may occur within the electron gun assembly.

The presence of a resistor between the internal coating of the bulb of a cathode ray tube and the final anode of its electron gun assembly reduces the quantity of energy which, on the occurrence of a flashover, can pass to circuits connected to the gun assembly. However, there exists astray capacitance formed between the combination of the second and fourth anodes, the anodes in the tube and thier immediate surroundings. In a tube with magnetic deflection, the principal contributor in the intermediate surroundings to this stray capacitance is the deflector yoke. Before a flashover this stray capacitance is charged to the full voltage of the high voltage supply. When a flashover occurs, the charge from this stray capacitance flows unimpeded into circuits connected to the gun assembly and, if the stray capacitance is large enough, can deliver sufficient energy to damage semiconductor devices. We have found that the presence of a resistor between the internal coating and the final anode contributes to this stray capacitance in a manner which depends mainly upon the physical size of the electrically conductive material of the resistor. For example, we have found that in one example of a cathode ray tube having a resistor with a conductive element deposited on the inner face of the neck of the envelope, the said stray capacitance is picofarads. A general description of this prior art cathode ray tube is given in United Kingdom patent specification No. 1,211,898.

It is accordingly an object of the present invention to provide a cathode ray tube in which the risk of damage to external circuitry by flashover within the tube is minimised.

SUMMARY OF THE INVENTION According to the present invention there is provided a cathode ray tube housing an electron gun assembly mounted within a sealed evacuated envelope formed substantially of electrically insulated material and having a first coating of electrically conductive material on part of its external surface and a second coating of electrically conductive material on part of its internal surface, the said conductive coatings forming a capacitor.

which is electrically connected to an electrode of the electron gun assembly by a resistor included in the said assembly.

Preferably the resistor has a conductive element which provides the resistance of the resistor and is supported by a rigid member formed of electrically insulating material.

In one preferred embodiment the said conductive element is in the form of a helix of resistive material, and the rigid member is a hollow cylinder of insulating material, the helix being coaxial with the cylinder and disposed on the internal or the external surface of the cylinder.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of a cathode ray tube embodying the invention, and

FIG. 2 is a longitudinal sectional view of part of the cathode ray tube of FIG. 1 on a larger scale.

DESCRIPTION OF PREFERRED EMBODIMENT In FIG. 1 there is shown a cathode ray tube 10 having an electron gun assembly 11 mounted within a sealed evacuated envelope 12 formed substantially of electrically insulating material and having a first coating 13 of, electrically conductive material on part of its external surface and a second coating 14 of electrically conductive material on part of its internal surface. The coatings 13 and 14 form a capacitor which is electrically connected to an electrode, which is the final anode 15, of the electron gun assembly 11 by a resistor 16 includcd in the assembly 11.

The electron gun assembly includes a thermionic cathode 17 mounted over a heater, a modulation electrode l8, and first, second and third anodes 19, 20 and 21 respectively, the secondanode 20 being connected directly to the final anode 15.

In operation, a modulation signal circuit is connected to apply a modulation signal between the modulation electrode 18 and the cathode 17, the modulation signal circuit supplying a voltage to the cathode 17 which voltage is varied to vary the intensity of the electron beam emitted by the cathode 17, the electrode 18 being maintained at a constant bias voltage. Furthermore, the anodes 19, 20 and 21 are connected to supply circuits which provide voltages enabling these anodes together with the final anode 15 to form and focus the electron beam to a point on the screen. Means, not shown, are also provided for deflecting the beam to desired positions, such means preferably being in the form of electromagnetic deflection coils located outside the envelope 12.

The first coating 13 on the envelope 12 consists of a layer of conductive graphite and extends over substantially the whole of the curved surface of a bulbous portion 22 of the envelope 12 which terminates at its wider end in a substantially flat face plate 23, one boundary of the coating 13 lying in a plane parallel to the face plate 23. A small part of the bulbous portion 22 of the envelope 12 is formed by an electrically conductive connector 24, and a clearance is left between the coating 13 and the connector 24 so that they are electrically insulated from one another.

The connector 24 makes electrical contact witha first portion 25 of the second coating 14 which portion 25 is in the form of a layer of conducting graphite,the coating 14 having a second portion 26 which is in the form of a layer of aluminium which makes electrical connection with the first portion 25 at a common boundary 27 lying in a plane parallel to the face plate 23.,The inner surface of the face plate 23 is coated with a layer 28 of electroluminescent material which is overlain by part of the second portion 26 of the coating 14. The first portion 25 of the coating 14 extends from the common boundary 27 to the smaller end of the bulbous portion 22 of the envelope 12 and some distance into a cylindrical neck 29 which forms the remaining por tion of the envelope 12,'the portion 25 ending in the neck 29 at a boundary 30 which lies in a plane parallel to the face plate 23 and is closer to the face plate 23 than is the resistor 16.

Details of the resistor 16 and the manner in which it is mounted in the envelope 12 are shown in FIG. 2.

The resistor 16 has a conductive element 31 which provides the resistance of the resistor 16 is supported by a rigid member 32 formed of electrically insulating material, for example, glass. The conductive element 31 is in the form of a helix of resistive material, for example a resistive material containing graphite as its primary constituent, and the rigid member 32 is a hollow cylinder, the helix being coaxial with the cylinder and disposed on the internal surface of the cylinder. A protective layer 33 of chromic oxide overlies the conductive element 31, but may be omitted.

Although the element 31 is shown disposed on the internal surface of the member 32, it may be disposed on the external surface of the member 32 with consequent rearrangement of the electrical contacts to be described below.

That end of the element 31 which is the nearer to the portion 25 of the coating 14 is electrically connected to it through a plurality of spring contacts 34, an outwardly directed flange 35,of a flanged metal member 36, and a further plurality of spring contacts 37, the contacts 34 and 37 being welded to the flange 35 and having their respective free ends in electrical contact with the said end of the element 31 and the portion 25 of the coating 14 as shown in FIG. 2. The conductive element 31 is terminated at each of its ends by respective short lengths 31 and 31 of low resistance graphite deposited around the internalcircumference of the rigid member 32.

The spring contacts 34 make electrical and physical contact with the length 31.

The flanged member 36 has an inwardly directed flange 38 which defines a circular aperture through which the electron beam passes in operation.

The other end of the element 31 is electrically connected to the final anode through a plurality of spring contacts 39 welded to an outwardly directed flange 40 of the anode 15. Thus the inner coating 14 is electrically connected to the anode 15 by the resistor. 16. The spring contacts 39 make electrical and physical contact with the length 31".

Alternatively, the lengths 31 and 31" may be omitted, and the spring contacts 34 and 39 make contact directly with the ends of the element 31'.

The flanged member 36 is secured to the flange 40 of the anode 15 by a plurality of insulating rods 41, for example of glass, having short lengths 42 of stiff wire embedded coaxially in their ends and welded to the flanges 35 and 40. Thus the resistor 16, which is mechanically supported by the spring contacts 39 and 34 which hold the rigid member 32 coaxial with the trajectory of the electron beam, is mounted in the electron gun assembly 11.

A tube getter 43 is also included in the electron gun assembly, being mounted on the flange 35 of the member 36.

The coatings 13 and 14 form a capacitor which can be used in a smoothing circuit for a high voltage supply circuit for use with the tube 10. This capacitor contains a charge during operation of the tube, the connector 24 i being supplied with a positive voltage, and the external coating 13 being connected to earth. The anodes 15 t and 20 are normally maintained at the same voltage as the internal coating 14 in operation, but may undergo a lowering of voltage if, as sometimes occurs when the electron beam constitutes a high emission current, the beam is partially intercepted by one or both of the anodes 15 and 20, these electrodes being electrically connected to the connector 24 only by the resistor 16.

A flashover from one of the anodes 15 and 20 to one or more of the other electrodes, even the heater, having lower voltages may occur within the electron gun assembly. When such a flashover does occur the resis tance between the said capacitor formed by the coatings l3 and 14 and one or more of the electrodes 18 to 21 is reduced, butthe resistance of the resistor 16 is chosen to be sufficient even in these circumstances to restrict the current flowing from the said capacitor to a value which will not damage any of the supply circuits or the modulation circuit, which may contain components such as solid state devices which are vulnerable to damage by high current levels, which are connected to the electrodes 18 to 21 in operation. For most circumstances the resistance provided by the element 31 should be at least 1000 ohms.

In another embodiment the said conductive element is in the form of a continuous film supported on an insulating base, and in a further embodiment the conductive element is in the form of a hollow cylinder. of conductive material having sufficient rigidity to support itself and thus constituting the resistor.

The presence of a resistor between the internal coating of the bulb of a cathode ray tube and the final anode of its electron gun assembly reduces the quantity of energy which, on the occurrence of a flashover, can pass to circuits connected to the gun assembly. How

ever, there exists a stray capacitance formed between the combination of the second and fourth anodes, the

anodes 20 and 15 in the tube shown in FIG. 1, and their immediate surroundings. In a tube with magnetic deflection, the principal contributor in the immediate surroundings to this stray capacitance is the deflector yoke. Before a flashover this stray capacitance is charged to the full voltage of, the high voltage supply connected to the high voltage connector corresponding to the connector 24. When a fla'shover occurs, the charge from this stray capacitance flows unimpeded into circuits connected to the gun assembly and, if the stray capacitance is large enough, can deliver sufficient energy to damage semiconductor devices. It has been found that the presence of a resistor between the internal coating and the final anode contributes to this stray capacitance in a manner which depends mainly upon the physical size of the electrically conductive material of the resistor. For example, it was found that in a cath ode ray tube having a resistor with a conductive element which is of the form of the element 31 but does not embody the present invention, the conductive element being deposited on the inner face of the neck 29 of the envelope 12, the said stray capacitance is picofarads, whereas in a cathode ray tube differingonly in that the resistoris constructed and arranged as described hereinbefore with reference to the resistor 16 the said stray capacitance is 3 picofarads We claim:

1. A cathode ray tube having an electron gun assembly mounted within a sealed evacuated envelope formed substantially of electrically insulating material and having a first coating of electrically conductive material on part of its external surface and a second coating of electrically conductive material on part of its internal surface, the coatings forming a capacitor which is electrically connected to the final anode of the electron gun assembly by a resistor included in the assembly, a small part of the envelope being formed by an electrically conductive connector, and a clearance being left between the external coating and the connector so that they are electrically insulated from one another, the connector making electrical contact with a first portion of the second coating which portion is in the form of a layer of conducting graphite, the internal coating having a second portion which is in the form of a layer of aluminium which makes electrical connection with the first portion at a common boundary lying in a plane, the said resistor having a conductive element which provides the resistance of the resistor and is supported by a rigid member formed of electrically insulating material and is in the form of a helix of resistive material, the rigid member being a hollow cylinder, the said helix being coaxial with the cylinder and disposed on the internal surface of the cylinder with a protective layer of chromic oxide overlying the conductive element, that end of the element which is nearer to the said first portion of the internal coating being electrically connected to it through a plurality of spring contacts, an outwardly directed flange of a flanged metal member, and a further plurality of spring contacts, the said contacts having their respective free ends in electrical contact with the said end of the element and the portion of the internal coating, and the other end of the element being electrically connected to the final anode through a further plurality of spring 

1. A cathode ray tube having an electron gun assembly mounted within a sealed evacuated envelope formed substantially of electrically insulating material and having a first coating of electrically conductive material on part of its external surface and a second coating of electrically conductive material on part of its internal surface, the coatings forming a capacitor which is electrically connected to the final anode of the electron gun assembly by a resistor included in the assembly, a small part of the envelope being formed by an electrically conductive connector, and a clearance being left between the external coating and the connector so that they are electrically insulated from one another, the connector making electrical contact with a first portion of the second coating which portion is in the form of a layer of conducting graphite, the internal coating having a second portion which is in the form of a layer of aluminium which makes electrical connection with the first portion at a common boundary lying in a plane, the said resistor having a conductive element which provides the resistance of the resistor and is supported by a rigid member formed of electrically insulating material and is in the form of a helix of resistive material, the rigid member being a hollow cylinder, the said helix being coaxial with the cylinder and disposed on the internal surface of the cylinder with a protective layer of chromic oxide overlying the conductive element, that end of the element which is nearer to the said first portion of the internal coating being electrically connected to it through a plurality of spring contacts, an outwardly directed flange of a flanged metal member, and a further plurality of spring contacts, the said contacts having their respective free ends in electrical contact with the said end of the element and the portion of the internal coating, and the other end of the element being electrically connected to the final anode through a further plurality of spring contacts. 